package main

import (
	"io"
	"bytes"
	"fmt"
	"reflect"
)

func f(out io.Writer) {
	var out io.Writer
	fmt.Println(reflect.TypeOf(out))

	if out != nil {
		out.Write([]byte("done!\n"))
	}
}

func main() {
	var buf *bytes.Buffer
	f(buf)
}
